Field of the Present Invention
The present invention relates generally to photovoltaic cells, and, in particular, to photovoltaic cells having improved surface geometries coated with layers doped with phosphors for both down-conversion and up-conversion.
Background
Photovoltaic solar cells are a well-known technology. The solar market is growing at a rapid speed; annual shipments of solar cell modules have been increasing at a rate of over 30% in the past few years.
As shown in FIG. 1, a conventional solar cell 10 includes an n-type semiconductor material 14 and a p-type semiconductor material 16. The n-type semiconductor material 14 is doped with impurity atoms to form an electrically negative material that already has a few electrons in its conduction band. The p-type semiconductor material 16 is doped to leave missing electrons, or holes, in its valence band. The junction 12 between the n- and p-type semiconductor materials 14,16 creates a voltage bias. When the solar cell absorbs incoming photons, electrons are caused to migrate toward the positive side of the junction 12 and holes toward the negative side, forming an electric current.
Unfortunately, the efficiency of conventional solar cells—that is, the proportion of the sunlight falling on the solar cell that is converted to electricity—is very low. Sunlight comes in many colors, combining low-energy infrared photons with high-energy ultraviolet photons and all the visible-light photons in between. The visible region of the solar spectrum, whose photons have wavelengths of between 400 nm and 700 nm and energies of between 3.1 eV and 1.77 eV, contains only about 43% of the irradiance. The ultraviolet region of the solar spectrum, whose photons have wavelengths less than 400 nm and energies of greater than 3.1 eV, contains about 5% of the total irradiance, while the infrared region, whose photons have wavelengths of greater than 700 nm and energies of less than 1.7 eV, contains about 52% of the irradiance. This is demonstrated in FIG. 2, which is a graphical representation of the relative photon energies present in sunlight under standardized conditions. In particular, solar radiation flux, measured in photons per second per meter squared, is plotted against photon energies for sunlight incident on the earth's surface under the well-established AM1.5 standard. Flux is utilized in FIG. 2 instead of irradiance (W/m2×nm) because flux is the only one-to-one relevant value when it comes to generation of carriers within the cell.
Each photovoltaic material responds to a narrow range of these energies, corresponding to its characteristic band gap, which is the amount of energy, expressed in electron volts (eV), that are required to kick an electron from a semiconductor's valence band (where electrons, bound to atoms, are plentiful) into its empty conduction band. Photons with energy lower than the band gap escape unabsorbed, while photons with higher energy are absorbed, but most of their energy is wasted as heat. Because band gaps are so limited, typical solar cells have efficiencies of no more than 25%, meaning most of the sunlight falling on them is not converted to electricity.
One common photovoltaic material is silicon (Si). FIG. 3 is a graphical representation illustrating the dependence of silicon absorption depth on light energy. As demonstrated in FIG. 3, Si is almost transparent to photons with energies lower than its bandgap. These photons do not contribute to the generation of electron-hole pairs. At an intermediate energy range of 3 eV>E>1.5 eV, Si absorbs light to a depth ranging from about 0.1 to 10 microns. In this range, Si has its maximum efficiency for solar cell applications. Electron-hole pairs are generated within the junction volume (for most cells) and are driven to the contacts by the built-in field. At energies higher than about 3 eV, the light is absorbed in the sub-surface area, 100 nm from the surface. Since carriers are generated near the surface and there is no built-in field to drive the carriers to the surface, a large proportion of these carriers recombine at surface states or within the subsurface layer, thereby resulting in minimal contribution to the generated current and much of the absorbed energy being converted to heat. In summary, Si utilizes about 45% of the incident light, assuming that the light is absorbed, minimal amount is reflected, and carriers are collected efficiently.
Because of these shortcomings, researchers in a number of different fields have pursued a variety of different approaches for improving the efficiency of solar cells. For example, materials researchers have proposed the use of different materials with different band gaps that can be stacked to capture photons with a wider range of energies. Such a “multijunction” solar cell includes a top junction that captures high-energy photons but allows lower-energy photons to pass through to one or more lower-band gap junctions below. Theoretically, efficiencies of 50% could be produced using such an approach. Unfortunately, it is very difficult to stack conventional materials, and matching materials with different crystal lattices is difficult and often impossible, and the actual efficiencies that have been produced thus far have been far less.
Another approach that has been proposed is the use of a “multi-band gap material,” wherein a single semiconductor produces multiple band gaps, thereby converting multiple spectral ranges to electricity. By replacing a few of the host atoms in a semiconductor alloy with nitrogen or oxygen atoms having a very different electronegativity, a split band gap can be produced, which in some materials produces a narrow band well below the normal conduction band. The presence of these two separate bands means the material efficiently absorbs photons of three different energies. The difference between the material's valence band and the lower of the split bands forms a first band gap, absorbing photons of a first energy level; the difference between the two split bands is a second band gap, absorbing photons of a second energy level; and the difference between the valence band and the upper conducting band forms a third band gap, absorbing photons of a third energy level. Such an approach could theoretically produce efficiencies in excess of 50%. However, research and development of this approach is still in its infancy, and unfortunately there are significant manufacturing hurdles to be crossed before such an approach will be able to find commercial success.
Another group of researchers, typically involved in the investigation and use of improved manufacturing techniques, has been exploring the use of optically-engineered surface geometries to minimize surface reflection and maximize absorption of light. Most or all of the efforts in this regard have focused on increasing the surface area of the material relative to the volume. For example, porous silicon is a skeleton of single crystalline silicon filled with randomly structured voids that result in a high surface to volume ratio. Thus, any junction built into the structure will have a much larger junction volume (depletion region) proportional to the surface area. However, a significant challenge with porous silicon is the high density of surface states within the porous structure, which leads to carrier recombination at the surface before collection by the electrodes. In other words, carrier removal to improve the efficiency of the solar cell has proven difficult to achieve.
Another group of researchers has been exploring the use of optically-engineered surface geometries to minimize surface reflection and maximize absorption of light. For example, U.S. Patent Application Pub. No. 2009/0295257 discloses the use of a variety of surface features, including an array of surface pyramids, an array of trenches, an array of corrugations, an array of crenulations, an array of nano-bowls, and combinations thereof. FIG. 4 is a schematic diagram, in isometric form, of a solar cell 11 having conventional corrugated trenches 20. However, while useful for better capturing or “trapping” light within the solar cell, these geometries are all still relatively crude, more frequently and a need exists for improved surface geometries for greater efficiencies. Furthermore, in part because researchers involved in optically engineering the surfaces of photovoltaic cells are rarely, if ever, materials researchers, the use of surface geometries has not been combined with other techniques to provide greater efficiency improvements. Thus, a need exists for still further improvements in surface geometries as well as in the use of optical engineering with other techniques.
Still another group of researchers, typically involved in materials research rather than manufacturing techniques, have proposed the use of a photon conversion layer, disposed on the top or light-incident side of the solar cell, for converting the wavelength of a portion of the photons received thereby to a different wavelength for better absorption thereof. For example, both U.S. Patent Application Pub. No. 2010/0012177 and U.S. Patent Application Pub. No. 2009/0255577 disclose a photon-conversion layer. However, researchers have only heretofore utilized a single layer, thereby performing down-conversion (from high-energy photons to low-energy photons) or up-conversion (from low-energy photons to high-energy photons) but not both, and no researcher has yet proposed the use of a second such layer on the opposite side of the solar cell. Furthermore, although efficiency improvements may have been achieved using such a single layer, the phosphors or other materials used to cause the energy (wavelength) conversion create additional reflectivity, thereby causing at least some amount of loss in efficiency because of the extra sunlight that is reflected from the surface of the cell. Thus, a need exists for the combination of a down-conversion layer, an up-conversion layer, or both with improved surface geometries to achieve less reflection in combination with an energy (wavelength) conversion process in a solar cell. This, and other needs, are addressed by one or more aspects of the present invention.